1. Technical Field
The present application relates to a porous silica material used as an acoustic propagation medium, and an optical microphone.
2. Description of the Related Art
A low-density porous silica material, commonly referred to as “silica aerogel”, has many pores and 90% by volume thereof consists of voids. The skeleton is formed by spherical silica particles of about some nm to some tens of nm connected together. A porous silica material has a low density and a low refractive index. The speed of sound propagating through a porous silica material is lower than the speed of sound through the air, i.e., 340 m/s. Therefore, it has been drawing attention as an acoustic propagation medium for various acoustic devices.
Conventionally, a porous silica material is manufactured by the following method. First, tetramethoxysilane (TMOS) represented by (Formula 1) below is mixed with a solvent such as ethanol to prepare a sol liquid.

Next, catalytic water is added to the sol liquid, and allowed to undergo hydrolysis and polycondensation reaction, thereby producing a wet gel. Then, the solution in the wet gel is replaced with a gas (dried), thereby obtaining a porous silica material.
When replacing the solution in the wet gel with a gas, the gel structure will be destroyed if the tensile stress based on the surface tension of the solution remaining in the pores is greater than the strength of the gel. In order to prevent this, supercritical drying is often used in the step of drying the wet gel. Also, a porous silica material with very little aging can be obtained by subjecting the gel to a hydrophobization process.
A non-patent document, Hidetomo Nagahara, Takashi Hashida, Masa-aki Suzuki, Masahiko Hashimoto, “Development of High-Sensitivity Ultrasonic Transducer in Air with Nanofoam Material,” Japanese Journal of Applied Physics, Vol. 44, No. 6B, pp. 4485-4489, 2005 discloses that a porous silica material is useful as an acoustic matching layer capable of efficiently taking in a sound wave, since the sound speed is low. It also discloses the relationship between the density of the porous silica material and the sound speed. As shown in FIG. 24, the lower the density, the lower the sound speed becomes.
Japanese Laid-Open Patent Publication No. 2009-85868 discloses an optical microphone as another application using a porous silica material. The optical microphone includes a photoacoustic propagation medium portion, and takes in a sound wave into the photoacoustic propagation medium portion to detect a distortion occurring in the photoacoustic propagation medium portion using light. Japanese Laid-Open Patent Publication No. 2009-85868 discloses that the photoacoustic propagation medium portion is suitable as the porous silica material.